In recent years, active efforts have been made to develop various types of high-energy-density batteries as the power source for compact-size portable devices as typified by mobile phones, nighttime power storage systems, household distributed electric energy storage systems based on photovoltaic power generation, electric energy storage systems for electric vehicles and the like. In particular, since lithium ion batteries have a volume energy density exceeding 350 Wh/L, and are superior in reliability in terms of safety and cycle performance, to lithium secondary batteries using metallic lithium as the negative electrode, the market for lithium ion batteries as the power source for compact portable devices is dramatically growing. Lithium ion batteries use a lithium-containing transition metal oxide such as LiCoO2 or LiMn2O4 as the positive electrode and a carbon-base material, typically graphite as the negative electrode. While the current challenge is to develop lithium on batteries with a further increased capacity, the approach of increasing the capacity of lithium ion batteries by improving the positive electrode oxides and negative electrode carbon-base materials, which are on practical use, has almost reached the limit. It is thus difficult to meet the demand for high energy density from the equipment side. Further, in a combination of a high-efficiency engine with an electric energy storage system (for example, in hybrid electric vehicles), or in a combination of a fuel cell with an electric energy storage system (for example, in fuel cell electric vehicles), constant output operation is essential in order for the engine or the fuel cell to operate at a maximum efficiency. Thus, in order to accommodate output fluctuations or energy regeneration on the load side, high-power discharging characteristics and high-rate charging characteristics are demanded on the electric energy storage system side. To comply with these demands, in the area of electric energy storage systems, research and development is performed on lithium ion capacitors to increase the output of lithium ion batteries featuring high energy density, or to increase the energy density of electric double-layer capacitors featuring high output.
On the other hand, the technology of electric energy storage devices such as lithium ion batteries and capacitors has placed attention on the technique that increases the capacity and voltage of electric energy storage devices by preloading (hereinafter referred to as predoping) active materials with lithium ions. When the predoping technique is applied to high-capacity materials such as insoluble and infusible substrates having a polyacene skeletal structure as described in Patent Document 1, Non-Patent Documents 1, 2, and 3, for example, electric energy storage devices can be designed so as to take full advantage (high capacity) of the predoping technique as described in Non-Patent Document 4. This makes it possible to meet the demand for higher energy density or output in the electric energy storage device. Predoping technique has long been used in practice. For example, Non-Patent Document 5 and Patent Document 2 disclose an electric energy storage device having high voltage and high capacity, in which an insoluble and infusible substrate having a polyacene skeletal structure, which is a negative electrode active material, is predoped with lithium. For lithium predoping, an electrochemical system in which an electrode to be predoped is used as the working electrode and lithium metal is used as the counter electrode is set up whereupon doping can be electrochemically performed. With this method, however, it is necessary to take the predoped electrode out of the electrochemical system and to reassemble it into a battery or capacitor. Therefore, as the practical predoping method, a method involving the steps of attaching a lithium metal foil to an active material-containing electrode to keep them in contact, injecting an electrolytic solution, and letting dope the active material with lithium has long been used. This method is effective for coin-type batteries having a small number of relatively thick electrodes. However, when the method is applied to multilayer structure batteries having a plurality of thin electrodes laminated or roll structure batteries, there arise problems including a complicated process and handling of thin lithium metal. There remains a need for a simple and practical predoping method.
As one solution to the above problems, Patent Documents 3 to 6 disclose a predoping method using a perforated current collector. For example, Patent Document 3 discloses an organic electrolyte cell characterized in that current collectors are provided with pores extending between front and back surfaces, a negative electrode active material is capable of reversibly carrying lithium ions, lithium originating from the negative electrode is carried by electrochemical contact with a lithium layer, the lithium layer is opposed to the negative or positive electrode, and an opposed area of the lithium layer is up to 40% of the area of the negative electrode. In this cell, electrode layers are formed on current collectors provided with through-pores for short-circuiting lithium metal and the negative electrode arranged in the cell, whereby lithium ions pass through the through-pores of the current collector after injection of electrolytic solution, whereby the negative electrode is wholly doped. Patent Document 3 discloses in Example an organic electrolyte cell using an expanded metal as the perforated current collector, LiCoO2 as the positive electrode active material, and an insoluble and infusible substrate having a polyacene skeletal structure as the negative electrode active material. The negative electrode active material may be easily predoped with lithium ions from the lithium metal arranged in the cell.
Also, it is disclosed that lithium metal powder is admixed in an electrode, or that lithium metal powder is uniformly dispersed on a negative electrode, a solution is injected therein, and local cells are constructed on the electrode, thereby absorbing lithium uniformly within the electrode, as described in Patent Document 7. Further, Patent Document 8 discloses a method involving admixing polymer-coated Li fine particles in a negative electrode material, forming a negative electrode, assembling a capacitor, impregnating the negative electrode with an electrolytic solution, dissolving away the polymer portion of the polymer-coated Li fine particles in the electrolytic solution to cause conduction (short-circuiting) between the Li metal and the carbon of the negative electrode, for thereby doping the carbon of the negative electrode with Li.
In all the above predoping techniques, once a battery or capacitor is assembled, an electrolytic solution is injected therein to start predoping. On the other hand, alternative methods are known, for example, a method of producing an electrode from a lithiated electrode material which is obtained by immersing an electrode material in a solution of n-butyl lithium in an organic solvent such as hexane, and reacting lithium with the electrode material (Patent Document 9); a method of intercalating lithium into graphite by reacting lithium kept in gas phase with graphite according to a process known as the Tow-Bulb process (Patent Document 10); and a method of mechanically alloying lithium by the mechanical alloying process (Patent Document 10).
As mentioned above, the predoping technique is important to the development efforts toward increasing the output of lithium ion cells or the energy density of capacitors, with a variety of predoping techniques being proposed. The predoping technique that is currently believed most practical is an electrochemical technique involving assembling a cell while an active material (electrode) and lithium are kept in direct contact or short-circuited via an electric circuit, and injecting an electrolytic solution therein, thereby achieving predoping. With this technique, however, a long time is necessary for achieving uniform doping throughout the material, metal lithium incorporated in the cell is not completely predoped, with some lithium being left, or lithium sites which are lost by predoping become vacancies to affect the internal resistance or other properties of the cell. Also, the use of perforated current collectors encounters the problem that the electrode material must be coated to the perforated collectors. The method of attaching a lithium metal foil to an active material-bearing electrode offers relatively high uniformity, but there are left many problems to be solved from the manufacture aspect, such as the problems of thickness accuracy and handling of a very thin lithium metal foil of up to 0.03 mm.
Also, a method of immersing an active material in a solution of alkyl lithium in an organic solvent such as hexane to directly predope the active material enables uniform predoping, but requires a large amount of lithium-containing reagent, as compared with the use of lithium metal as the lithium source, and very cumbersome steps of taking out the active material and separating the residual reagent at the end of reaction. Further, the doping techniques based on the Tow-Bulb process (gas phase) and mechanical alloying (solid phase) are difficult to implement in practice because the conditions are complex, special large-size equipment are necessary, and there are fatal problems including exposure of the material to be predoped to high temperature and breakage of material structure by grinding with violent forces.
As means for solving the above-mentioned problems, Patent Document 11 proposes a method of predoping a material with lithium by kneading and mixing a lithium-dopable material with lithium metal in the presence of a specific solvent for thereby simply predoping the material with lithium in the electrode preparing step. Patent Document 11 discloses in Example that when lithium metal foil of 0.03 mm thick is kneaded and mixed with polycyclic aromatic hydrocarbons (PAHs) obtained from thermal reaction of coal-based isotropic pitch at 680° C. and acetylene black in the presence of propylene carbonate, the lithium metal foil disappears, and the resulting mixture containing PAHs has a potential of 129 mV versus Li metal. Patent Document 11 describes that polycyclic aromatic hydrocarbons having a hydrogen/carbon atomic ratio of 0.05 to 0.5, such as insoluble/infusible substrates containing polyacene skeletal structure, carbon base materials, graphite base materials, conductive polymers, tin or tin oxide, silicon or silicon oxide, or the like may be used as the material to which the predoping method is applicable, but specifically describes nowhere the predoping conditions for each of these materials (e.g., silicon or silicon oxide) other than the polycyclic aromatic hydrocarbons and the predoped electrodes thereof.
On the other hand, as alluded to previously, efforts for improvements toward a higher capacity have been made by mainly using carbon materials such as graphite as the negative electrode material in lithium ion cells. However, the attempt to increase the capacity of carbon materials has almost reached the limit, and new high-capacity negative electrode materials of the next generation are proposed. One promising candidate of the next generation high-capacity negative electrode materials is a silicon-base material, with active development efforts being devoted thereon. For example, Patent Document 12 discloses, as the silicon-base material having a high capacity and improved cycle performance, silicon (Si), particles of composite structure having nanoparticles of silicon (Si) dispersed in a silicon-base compound, and a material obtained by covering the surface of silicon oxide of the general formula: SiOx (wherein 0.5≦x<1.6) with a graphite coating having a specific structure.
Also, Patent Document 13 discloses a negative electrode material for nonaqueous electrolyte secondary batteries, which is prepared by coating the surface of silicon-silicon oxide composite particles with a carbon coating, mixing the coated silicon-silicon oxide composite particles with a lithium dopant such as lithium hydride or lithium aluminum hydride, and heating the mixture for thereby doping the carbon-coated particles with lithium in the form of a compound, typically Li2SiO3. There are left some problems, for example, high-temperature heat treatment at 600° C. is necessary.